The present invention relates generally to the calendering of webs, such as the calendering of paper webs in machine-finishing operations.
In particular, the invention relates to web calendering wherein the web to be calendered is passed through one or more calendering nips formed between calender rolls, in which nips the web is pressed at a suitable linear load, and wherein at least the extreme or outer rolls are provided with internally situated means for varying or adjusting the roll crowns.
The invention further relates to apparatus for carrying out the calendering method including a machine stack comprising a calender stack including at least two, and preferably more, calender rolls which form one or more calendering nips between each other and in which an outer roll or rolls are provided with internally situated devices for varying or adjusting the crowns of the rolls.
Although the invention is described below in connection with paper webs, it will be understood that the method and apparatus of the invention are also suitable for use in the calendering of other types of webs, generally but not necessarily fibrous webs.
Paper coming from the drying section of a paper machine is usually not yet suitable for its intended purpose but, rather, requires various additional treatment steps.
An important post-manufacturing treatment of paper is calendering which, when performed as a separate working step, is generally referred to as machine-calendering or super-calendering. In machine calendering, the web is passed through one or more press zones or nips formed by rolls having hard and smooth surfaces. In the case where the web is passed through several nips, the calender rolls are usually journalled one above the other so that they can freely move in the vertical direction with respect to the bottom roll which is mounted with a fixed axis of rotation, to thereby form a multi-roll vertical calender
Important goals of calendering are to provide the paper with the desired smoothness and glaze and to adjust the thickness and bulk of the paper to desired levels. A related object is to equalize the thickness of the web in the transverse direction so that wound rolls of the web are even. Other functions are carried out by calendering, as are well known.
The calendering of webs have been compared to the ironing of cloth by means of a steam iron in that pressure, temperature and moisture are important factors along with the nature of the contact of the web with the calender rolls. The basic nature of the work performed by a calender or machine stack is rolling friction whereby deformation in the web is accomplished mainly by compression forces.
More particularly, the effectiveness of calendering on a web depends on a great number of factors, notably the surface pressure present in the press zone, which depends on the linear load in the nip, on the diameters of the calender rolls, on the thickness of the web, on the number of nips, and the temperature of the rolls, on the moisture of the paper and the distribution of the moisture in the cross-section of the paper, and on the speed of the machine.
The surface pressure present in the press zone is an important variable in the calendering operation. The higher the pressure, the higher is the effect of the calendering on the thickness and smoothness of the paper. On the other hand, an excessively high pressure may damage the web. For those paper qualities that require intensive calendering, the surface pressure is generally in the range of between about 20 to 50 MPa in the lowermost calendering nip. The number of nips is also significant.
The temperature of the rolls also affects the calendering operation. A high temperature will improve the smoothness of the paper and it is conventional to heat some of the calender rolls by steam or heated water.
The water content of the paper is also an important factor in calendering. Generally, increased water content in the paper web improves its smoothness but reduces the thickness, brightness and opacity of the paper. An excessively high water content causes the web to blacken due to the crushing of the fibers in the regions where they are linked together. Generally, calendering should be performed with the paper having a water content of between about 5 to 8%, although the calendering of newsprint as well as certain other printing papers which contain groundwood pulp can be performed with a water content of up to 9%.
It is conventional to dry certain types of cardboard to an excessive dryness whereupon the same are moistened at the calender to a desired water content. The water is added to the surface of the board either by means of a mist jet before the first nip or by means of a water doctor placed on one or more calender rolls. The water moistens only the surface layer of the board whereby it is possible to obtain a high degree of smoothness without compressing the board to an undue thinness.
The effect of calendering on a web also depends on the speed of the machine. Generally, the effectiveness of the calendering is reduced as the machine speed increases. This decrease in effectiveness can be compensated for by increasing the linear load in the nips, by increasing the number of nips, or by raising the temperature.
Moreover, the calendering of webs in high speed paper machines often results in detrimental barring of the web which is difficult to avoid. Specifically, patterns of transverse depressions or bars are formed in the web which are clearly visible and which repeat at regular intervals. Such bar patterns are also clearly visible in the thickness profile of the web in the machine direction. Barring results from oscillations of the calender which cause variations in the linear load in the nips.
Ideal conditions in which the linear load across the web is constant in each nip of the calender and in which the properties of the web being introduced into the calender, such as thickness, density, moisture, formation and the like, are uniform both in the longitudinal as well as in the transverse direction, never actually occur in practice. Thus, in practice, the calender rolls cannot be ground so as to be perfectly straight nor does the convexity curve of the rolls precisely follow their deflection curve. Variations in the properties of the paper web result from both the wet end as well as from the drying section of the paper machine. In order to compensate for these practical problems, adjustments must be made in the operation of the calender. One possibility is the adjustment of the temperatures of the calender rolls at different portions of the web along its transverse dimension.
If a region of the web in the machine direction is thicker than other regions, an increased linear load will exist in the calender nip at this region. In this manner, additional heat is generated in this region relative to other transverse regions so that the temperature of the calender rolls is elevated whereby the diameter of the rolls increases due to thermal expansion to cause even further generation of heat. Thus, the calender normally has a tendency to self-correct the transverse thickness profile of the web. Such self-correction is, however, generally not sufficient and corrections in the thickness profile must be made by other provisions, such as localized heating or cooling of the calender rolls.
Arrangements are known in the prior art for heating the calender rolls by electromagnetic induction whereby a magnetic flux is externally applied to the mantle of the calender roll by means of a magnetic shoe device spaced from the calender roll mantle by an air gap. Magnetic flux induces eddy currents in the roll mantle which in turn generate heat in the mantle due to the electrical resistance of the mantle. Such known magnetic shoe devices include several core components situated in side-by-side relationship which can be adjusted to in turn adjust the heating effect of the cores in the axial direction of the calender roll. With respect to the technique for induction heating of calender rolls, reference is made by way of example to Finnish patent application Nos. 812697, 820733, 821838 and 824281. Reference is also made to Finnish application Nos. 833589 and 843412, assigned to the assignee of the instant application.